NO884629L - QUICK RECYCLABLE PTFE AND A PROCEDURE FOR ITS MANUFACTURING. - Google Patents

Description

The present invention relates to porous polytetrafluoroethylene material (hereinafter referred to as PTFE) which has the property of rapid recycling and a method for the production of these materials. Recovery in connection with this document refers to the recovery of deformation, and especially stretching. The microstructure of the porous PTFE material consists of knots connected by fibrils, where almost all the fibrils have a bent or wavy appearance. Objects made from these materials are particularly suitable for use in the medical field, but are also useful in other fields, such as filtration and cloth or fabric applications.

Strong, porous PTFE products and their method of manufacture were first described in US patent 3,953,566. Such products have found wide acceptance in many areas. They are used in the medical field as replacements for veins and arteries, as overlap materials, as sutures and as ligaments. They also have useful applications in the areas of waterproof and breathable clothing, filtration, sealants and packaging, and in threads and filaments for weaving and sewing. These products have a microstructure with knots connected by fibrils.

US Patent 4,443,511 relates to a method for the manufacture of a laminated fabric composed partly of porous PTFE which has improved elastic properties. However, this patent only discusses how to make a stretchable laminated article, but does not show how to provide porous PTFE with a property of rapid recyclability.

According to the present invention, there is provided a porous shaped article consisting mainly of PTFE, which article has a microstructure of knots interconnected by fibrils, which article has a rapid recovery of more than about 5.5$.

The present invention also provides an extruded, expanded article, heat treated above its crystalline melting point, made from a material consisting essentially of polytetrafluoroethylene (PTFE), which porous PTFE has then been compressed at least 50% in the direction in which rapidly recoverable properties are desired, clamped and heated.

The products of the present invention are preferably made by taking shaped articles of porous PTFE which have been expanded or stretched by stretching and have been heated above their crystalline melting point, compressing these articles in a direction parallel to but opposite to that in which they was expanded by stretching, holding the objects and heating them to a temperature above their crystalline melting point for a period of time, allowing them to cool, removing the stress and re-stretching them in the direction of the original stretching to their approximate original length.

The invention will now be described in more detail, through an example, with reference to the attached drawings where: Figure 1 is a schematic representation of the microstructure for porous PTFE known from before;

Figure 2 is a schematic representation of the microstructure of the PTFE material according to the present invention;

Figure 3 is a photomicrograph (taken at 500 times magnification) of the surface of the prior art PTFE material made in accordance with US Patent No. 3,953,566 which was stretched in one direction;

figure 4 is a photomicrograph (taken with 500 times magnification) of the surface of the PTFE material according to the present invention,

figure 5 is a photomicrograph (taken at 20 times magnification) taken of the wall cross-section of the PTFE material according to the prior art;

figure 6 is a photomicrograph (taken at 20 times magnification) of the cross-section of the wall for the PTFE material manufactured in accordance with the present invention;

figure 7 is a schematic representation of a compression procedure for example 4;

Figures 8 and 9 are photomicrographs (taken at 500 times magnification of the wall cross-section for the PTFE material according to the prior art, and

figures 10, 11, 12 and 13 are microphotographs (taken with 500 times magnification) of the cross-section of the wall for the PTFE material described in example 1, table 1, test specimens 1, 2 and 3 and 4, respectively.

A method for the production of porous PTFE products which have a desired rapid recycling property has been provided. The porous products manufactured in accordance with the invention are distinguished from the previously known porous PTFE products in that they have a rapid recovery property greater than about 5.5$, i.e. they have a spring-like property. Furthermore, articles manufactured by the present invention can undergo repeated applications with tensile loads and exhibit approximately the same rapid recovery after each successive load application and removal cycle.

Quick recovery as defined herein is the difference between the stretched length of the material and the recovered length with respect to the recovered length. The stretched length is the length of the material under tensile load and the recovered length is the length of the material measured 5 seconds after lifting the tensile load.

An advantage that rapidly recoverable PTFE tubes manufactured in accordance with the present invention have is the improved bending characteristics that are exhibited over the previous porous PTFE tubes. These pipes also show improved resistance to buckling, constriction or collapsing during bending.

The porous PTFE material which forms the precursor to this invention can be produced by the method described in US patent no. 3,953,566. Using this method, a liquid lubricant is mixed with a commercially available powder of PTFE, and the mixture is extruded by a piston type extruder or other type of extruder. The material is then expanded by rapid stretching either uniaxially, biaxially or multiaxially after the liquid lubricant has been removed from it.

After stretching, the material is heated while held to a temperature above the crystalline melting point of the polymer and held there for a period of time. The time and temperature will vary depending on the amount of material being heated. In general, the higher the temperature used, the shorter the time. A strong, porous material is produced. This material which has undergone uniaxial expansion has a microstructure of nodules interconnected by fibrils which are mainly parallel and straight.

While this invention can be used with products that have been biaxially or multiaxially stretched, the description that follows will refer to a uniaxially stretched article.

Figure 1 is a schematic plan view of a section of a uniaxially expanded PTFE material produced using the techniques described in US patent 3,953,566. This section, when viewed under a microscope, is depicted as having many nodules 2 associated with many fibrils 4. This shows the microstructure in which the longitudinal axes of the fibrils are all substantially parallel to the direction of extension.

This precursor material is compressed in the direction parallel to, but opposite to, the direction in which it was originally expanded by stretching. The amount of compression depends on the nature of the shaped object and the desired product. In one embodiment, the shaped article is compressed up to the point just before it begins to wrinkle or shrink. In this embodiment, the shaped object is compressed to less than about half of its original length and preferably to less than about a quarter of its original length. A second embodiment continues the compression so that wrinkles or puckers are intentionally created. In a third embodiment, the shaped object can be compressed to a lesser extent so that it is less than its original length, but greater than half of its original length. In this third case, the rapid recovery property is greater than that of the precursor object, but less than that of a more compressed object.

The percentage compression is calculated from the following equation:

where 10 is the original length of the precursor material and l is the compressed length.

The material is then retained in its compressed state and heated in a furnace at a furnace temperature in the range of 100° to 400°C with the most preferred furnace temperature being about 380°C. The heating can be done with a large number of types of furnaces. The time period during which the Compressed or compressed articles heated can vary, but as a rule, the lower the temperature the longer the article must be held in. The time required for heating can also vary depending on the oven temperature, oven type and material mass.

While it is advantageous to use a heating step, the material exhibits the rapid recovery property if compressed and held at room temperatures, but it must be held for long periods of time, at least several days.

After the compressed material has been heated, it is advantageously cooled to about 23°C. The cooling can take place through natural air currents, or by forced cooling. After cooling, the tension is removed. Removing the clamping before cooling the compressed porous PTFE article may cause the article to lose some of its quick-recovery properties. The material is then re-stretched in the direction of the original stretching to approximately its original length.

While the foregoing has described the starting material for this invention as porous PTFE materials that have been heated above their crystalline melting point, porous PTFE materials that have not been heated may also be used. However, these materials do not demonstrate the range of rapid recovery found in the heat treated materials, but they do demonstrate rapid recovery greater than that exhibited by the precursor PTFE materials.

PTFE fibrils of any length in the article of this invention demonstrate the rapid recovery phenomenon; preferably, however, the filaments in the article according to this invention have an average length of less than 100 p.

The porous PTFE with rapid recovery is best understood by referring to the attached drawings. Figure 2 is a schematic plan view of a section of expanded PTFE manufactured in accordance with the present invention when viewed under a microscope. In this, knots 12 are connected with fibrils 14. It is clearly shown that, unlike the fibrils in Figure 1, essentially all the fibrils in Figure 2 have a bent or wavy appearance. Figures 3 and 4 are microphotographs of PTFE materials manufactured in accordance with the instructions according to the known technique and that manufactured in accordance with the present invention respectively. Figure 3 is a photomicrograph i of the precursor tube which was subsequently compressed, held, heat treated and re-stretched in the direction of the original stretch to approximately its marginal length, as shown in Figure 4.

Although the microstructure, in particular essentially all of the fibrils, undergoes a change in appearance, the overall appearance to the naked eye remains essentially unchanged. Figure 5 is a photomicrograph of the wall cross-section of the precursor PTFE tube material. Figure 6 is a photomicrograph of the cross-section of the wall for PTFE pipe material treated in accordance with the steps according to the present invention. This product shows smooth surfaces similar to those found in the precursor materials. One embodiment of the present invention is that in which the macrostructure (external surface and, for a pipe, external and internal surfaces) of the material does not demonstrate any apparent visible change in appearance. In an alternative embodiment, the outer surface and/or inner surface of the PTFE material can be modified to produce a corrected or rough surface.

Finished products manufactured in accordance with the method of the present invention may include items such as films, membranes, plates, tubes, rods and filaments. Sheets can also be manufactured by slitting pipes lengthwise. Porous fast-recyclable materials, including tubes, films, membranes or sheets, maintain their hydrophobic nature and are also permeable to water vapor, which makes them waterproof and breathable. They can be laminated, impregnated and bonded to other materials and substances to provide compound or composite structures that have quick recovery properties in addition to the known properties of expanded PTFE. These quick-recyclable PTFE articles are useful in the medical field for applications such as vascular transplants, in cloth or fabric applications, as well as in the fields of filtration.

An Instron tensile tester was used for all testing. All tests were performed at a temperature of 23°C ± 2°C. Each sample was cut in half transversely so that one part could be used to determine the maximum tensile force and the other to determine the rapid recovery. A crosshead speed of 500 mm/min and initial grip separation of 150 mm was used for all tests.

For specimens too small to accommodate the 150 mm grip separation and allow sufficient material inside the grippers to prevent the specimen from dropping inside the grippers, other combinations of crosshead speed and gripper separation may be used provided the ratio of the crosshead speed to the first gripper separation is equal to 3.33 minutes — 1. Keeping the ratio of crosshead speed to first grip separation constant, all tensile tests were performed at a strain rate of 333$/minute.

First, the maximum tensile force on one half of the test material was determined. This was done on the Instron tester using standard pneumatically actuated cord grippers. Fractures in the specimens occurred at a distance from the edges of the gripping devices. The maximum tensile force was measured for pipes, rods and plates for each individual specimen. For the plate specimen, the plate was folded in half lengthwise and placed between the cord grippers. To apply 1% of the maximum tensile force, the cord grippers were removed and replaced with pneumatically actuated flat grippers with rubber surfaces. The other half of the specimen (in either tube or rod form, or unfolded plate) was placed between these grippers and a tensile load of 1% of the previously determined breaking force or maximum tensile force was applied using identical grip separation and load rating as described above.

The length (lt) of the specimen while below 1% of the maximum tensile force was determined as the grip separation distance. Alternatively, the length (lt) can be measured from the Instron tester's chart recording device.

Immediately after reaching a tensile load equivalent to 156 of the maximum tensile force, the lower Instron gripper was quickly released to allow the specimen to recover. Five seconds after release of the stretch, the actual length (lr) of the sample that had been between the grippers of the testing device was measured.

The percentage rapid recovery was then calculated from the following equation:

where lt was the length of the test specimen under the tensile load and lr was the length of the test specimen measured five seconds after release of the tensile load.

Wall thickness measurements for the precursor tubes were taken by cutting a 2.5 cm long sample from the end of the tube sample with a razor blade. This sample was then uniformly adjusted over a stainless steel mandrel with an outside diameter corresponding to the inside diameter of the pipe. Thickness measurement was carried out using a profile projector to measure the distance from the outer surface of the mandrel to the outer surface of the cut end of the pipe sample. The measurements were made at 3 locations approximately 120° apart around the circumference of the pipe. The wall thickness of the sample was taken as the mean value of these three measurements.

The fibril length of the test samples in the following examples was determined by photographing the surface of the sample at 200x magnification. Two parallel lines were drawn 12 mm above and below the longitudinal center line of the photographs, parallel to the direction of the fibrils. Following the upper edge of the upper line and starting from the left edge of the photograph, the distance from the right edge of the first distinct knot to the left edge of the second distinct knot was measured as the first fibril length. Measurements were made using divisions referenced to a scale that took into account the magnification factor.

Five consecutive fibril length measurements were made in this way along the drawn line. The photograph was rotated 180° and 5 consecutive fibril length measurements were taken from the left edge of photographs along the upper edge of the second drawn line. The mean fibril length for the sample was taken to be the mean of ten photographic measurements.

Both wall thickness and fibril length measurements were made on the precursor tubes before the inventive treatment steps were applied.

The following examples describing methods and products according to the present invention are illustrative only and are not intended to limit the scope of the present invention in any way whatsoever.

EXAMPLE 1

FAST RECYCLABLE EXPANDED PTFE PIPES

CD123 fine powder PTFE resin (supplied from ICI Americas) was mixed with 150 cm<3>ISOPAR M (Trademark), odorless solvent (obtained from Exxon Corporation), per pounds of PTFE resin. The mixture was compressed into a tubular blank, heated to about 60°C and extruded into tubes in a piston extruder at a reduction ratio of about 240:1. The lubricant was removed from the extrudate by drying in a forced convection oven at 250°C for thirty minutes. The tubes were then expanded by stretching using the stretching technology shown in US patent 3,953,566. Test tubes 1,2,3 and 5 in table 1 were expanded 8.4:1, at a rate of about 50$ per second in a forced convection oven at a temperature of 290°C. These tubes had fibril lengths of about 35 microns. Sample tube 4 was expanded 2.3:1, at a rate of about $160 per second in a forced convection oven at a temperature of 290°C. These tubes had a fibril length of about 10 microns. All tubes, except sample 5, were then heat treated in a gravity convection air oven for 90 seconds at 393°C. The Alel tubes had an internal diameter of 10 mm.

One tube of each type was retained as a control sample. Percent rapid recovery measurements for the control or precursor tube are shown compared to tubes treated with the method of the present invention in Table 1. The tubes were fitted over stainless steel mandrels with an outer diameter of 10 mm. One end of each tube was attached to its mandrel with a retaining thread. The free end of each pipe sample was pushed by hand in the longitudinal direction against the fixed end of the pipe, thus compressing the pipe sample in the longitudinal direction. The percentage compression for each sample was calculated using the formula described above and is shown in Table 1. Each pipe sample was compacted or compacted uniformly without wrinkling or otherwise disturbing the exterior surface. The free end of each tube was attached to the mandrel with a second retaining thread. Each tube and mandrel assembly was placed in a gravity convection air oven for a predetermined time and oven temperature, both parameters being listed in Table 1. Each tube and mandrel assembly was removed from the oven and cooled to about 23°C. The retaining wire for each device was removed and the tubes were removed from their mandrels. Each sample was re-stretched in the direction of the original stretch to approximately its original length and allowed to recover for at least one hour. Percent rapid recovery was calculated for each tube using the equation described earlier. The results for the percentage rapid recovery are summarized in table 1.

EXAMPLE 2

RODS

A 0.635 cm nominal diameter rod of GORE-TEX (Trade Mark) Joint Sealant, commercially available from W.L. Gore and Associates, Inc., Elkton, MD, was used as a precursor to porous PTFE material which was treated by the process of the present invention. This material had not been heated above its crystalline melting point.

A portion of the test sample was cut off and retained as a control to determine the percent rapid recovery of the precursor material. A second sample of 300 mm length was placed in a thin-walled stainless steel tube of 0.53 cm internal diameter and manually compressed to achieve a compression of 67%. This compression was maintained by using a plug-in tension. The assembly was heated in a gravity convection air oven to an oven temperature of about 300°C for 15 minutes while maintaining the sample in its compressed state. The unit was removed from the oven and cooled to approximately 23°C. The clamp was removed and the sample was removed from the stainless steel tube. The specimen was re-stretched in the direction of the original stretch by hand to approximately its original length and allowed to recover for more than 1 hour. Percent rapid recovery measurements were made on both the control samples and the samples treated according to the inventive steps as described above. The measurements and calculations were made using the percentage rapid recovery as defined in the description. The results are shown in table 2.

EXAMPLE 3

REPETITION CAPACITY

A rapid recovery pipe (Specimen 1) from Example 1 was stretched 10 times to determine the range of percent rapid recovery for a specimen subjected to repeated stretching. The specimen was placed between the grippers of an Instron tensile fixture with a gripper separation of 150 mm. The crosshead speed was set at 500mm/minute to give a strain or strain rate of 333$ per minute. minute. The Instron machine was programmed to stop its crosshead movement upon reaching 0.53 kilograms of tensile load similar to that of Example 1, Specimen 1. The length (lt) of the specimen while below 1 percent of maximum tensile force was determined by measuring the distance between the gripping devices. Alternatively, length (lt) can be measured from the Instron instrument's waveform recorder. When the crosshead stopped, the lower gripper on the Instron instrument was released, which released the sample and allowed it to regain its length. Four seconds after the specimen was released from the lower gripper, the actual length lr of the specimen was measured and recorded. The cross head was returned to its initial position to again provide a gripping separation of 150 mm. The specimen was again clamped between the lower grippers of the Instron tester at the same point on the specimen. 60 seconds after the first stretch application, 0.53 kg of stretch was again applied and the entire test was repeated. This was done 10 times in total, with lt and lrm measured and recorded each time. The percentage rapid recovery was determined as previously defined for each stretch application. The results are reproduced in table 3.

EXAMPLE 4

FLAT PLATE

Flat rectangular sheets of expanded PTFE were produced by longitudinally slitting a tube with an internal diameter of 10 mm manufactured in the same manner as the precursor tube of Example 1, sample 1. One of the test sheets was retained as a control sample to determine its percentage rapid recovery. Reference is made to figure 7 where a second test plate 21 was clamped between two thin plates 27 and 29 of stainless steel and separated by about 0.9 mm from each other. An end edge 23 of the sample plate 21 was restrained from movement. The opposite end edge 25 was moved towards the fixed end edge using a plate 31 of stainless steel to thus compress the material sheet in the direction of the fibrils, i.e. in a direction parallel to, but opposite to, the direction in which it was stretched. The material was compressed 83%. The compressed sample was held and the assembly was heated in a gravity convection air oven at a temperature of about 380°C for about 5 minutes. After removal from the oven, the unit was cooled to about 23°C and the plate sample was removed from between said plates. The sample was again stretched in the direction of the original stretch to approximately its original length and allowed to recover for more than 1 hour. The percentage rapid recovery was measured and calculated using the formula previously described. The results for both the sample according to the invention and the control are shown in table 4.

EXAMPLE 5

FAST RECYCLABLE EXPANDED PTFE PIPES

AND FIBRIL MEASUREMENTS

Specimens 1,2,3 and 4 of Example 1 were further evaluated to estimate the amount of bending imparted to the fibrils by the rapid recovery process. SEM photographs taken at 500X magnification (Figures 8, 9, 10, 11, 12 and 13) were taken for the wall cross-section of each specimen. Figures 8 and 9 are photomicrographs of precursor samples having fibril lengths of 10 micrometers and 30 micrometers respectively. Each specimen was prepared by cutting out a segment and allowing it to relax for a sufficient period of time, in this case 24 hours. A relaxed state is the one where the test segment exists without tensile or compressive influence. Each sample was set up and maintained in its relaxed state.

SEM photographs were taken at magnification levels of 500X. Two important factors in determining the level of magnification include resolution and fibril sample handling. The photograph taken showed between five and ten complete sequential fibril sets.

The photomicrographs were marked with two parallel drawn lines placed 24 mm apart, approximately centered on the photograph and oriented so that the lines were substantially parallel to the direction of the fibrils. Moving from left to right along the upper dashed line, the distance "H" between knots was determined to be the distance between the knot attachment points of the first distinct fibril closest to the drawn line. A distinct fibril is one whose complete length can be distinguished visually. Vertical displacement, a distance "V" was then measured as the perpendicular length from the distance "H" to the farthest point on the fibril. If the fibril crossed the distance "H" one or more times, then the distance "V" was determined to be the sum of the maximum perpendicular "V" measurements. The ratio of V/H was calculated for the fibril. Moving to the right along the solid line, the "V" and "H" measurements were determined for four additional fibrils. The photograph was rotated 180° and the process was repeated for four additional fibrils. The mean values for "V" and "H" and V/H were calculated for all ten fibrils examined. The measurement results are summarized in table 5.

Claims (8)

1. Porous, shaped article consisting mainly of polytetrafluoroethylene (PTFE), which article has a microstructure of knots (12) connected by fibrils (14), characterized in that the article has a rapid recovery of more than about 5.5$. 2. Item according to claim 1, characterized in that essentially all the fibrils (14) have a bent appearance. 3. Item according to claim 1 or 2, characterized in that it is in the form of a tube with smooth internal and external surfaces. 4. Process for the manufacture of a porous, shaped object according to claims 1 to 3, characterized in that a preformed object is extruded from a mixture of coagulated dispersion of PTFE and liquid lubricant, that the material is expanded after removal of the liquid lubricant by stretching the material in the longitudinal direction and maintaining the material at a temperature between 35°C and the crystalline melting point during stretching, compressing the extruded and expanded PTFE in the direction of the fibrils to reduce its size, maintaining the PTFE article in its compressed state, and heating the compressed PTFE article . 5. Method according to claim 4, characterized in that the material is compressed at least 60$, or 75$, or 90$, in the direction of the desired rapidly recoverable characteristic. 6. Method according to claim 4, characterized in that the compressed PTFE material is heated to a temperature above its crystalline melting point. 7. Method for manufacturing a porous shaped object according to claim 1, in the form of a tube of PTFE, characterized by: a) the tube is compressed and held to less than about 50% of its original length; b) it is heated at an elevated temperature over a period of time; c) it is allowed to cool; and d) the pipe is stretched to approximately its original length. 8. Method according to claim 6, characterized in that the compressed PTFE material is not heated.